The plant community usually is the largest visible part of an ecosystem, and often both the community and the ecosystem are named for the dominant plants present—that is, the plants that, by virtue of their size or numbers, modify and control the environment. The community is not a haphazard collection of organisms, but consists of populations of individuals whose tolerance ranges—the range of environmental conditions in which individuals of a particular species will grow—match those of the site.
Even the most stable ecosystems are in constant, normal flux. One of the easiest ecological processes to observe is succession, the change in the composition of the vegetation of a particular site over time. Two kinds occur. Primary succession takes place on newly exposed surfaces such as might appear after a volcanic eruption or following a rockslide in the mountains. Secondary succession occurs when vegetation is removed from land and new kinds of plants return to colonize the bare ground. On sites undergoing primary succession, no soil is present, and vegetation and soil develop concurrently. In secondary succession, vegetation develops on soil already in place, but the soil changes over time as the new colonizers develop new communities above, and below ground.
Weedy annuals are the usual pioneer species that colonize bare ground. They are “generalist” species with broad tolerance ranges and by their growth change conditions at the site, making possible the development of communities of other species. Productivity in the early stages is high, but as species richness and total biomass increase, productivity decreases (the reason why agricultural ecosystems are kept in an early successional stage).
Disturbances are necessary to sustain ecosystems and are the mechanisms by which diversity in kinds and ages of species and habitats is maintained. Some disturbances are large scale and rare (a tornado or hurricane, for example), but most are small and frequent (the blowdown of a few trees opening a hole in the forest canopy or rodent burrowing that brings underlying soil to the surface and destroys existing ground cover).